Linear low density polyethylenes exhibit extremely desirable properties in film products. However, processing characteristics of the linear low density polyethylene (LLDPE) are not equal to the inherent properties of the resin itself. Commercial application of LLDPE resins is limited partly because of extrusion problems, including high back pressure and power consumption due to extremely high shear viscosity, and partly because of film surface defects due to melt fracture.
Accordingly, processing aids have been employed in conjunction with the linear low density polyethylene resins to improve those processing characteristics. For example, the extrudability of film grade LLDPE with narrow molecular weight distribution can be improved by blending. Binary blends containing high amounts (10 to 15%) of polypropylene or polystyrene with the LLDPE exhibit improved processing characteristics, but the high levels (10 to 15%) of polypropylene or polystyrene reduce the properties of the base LLDPE so greatly that little or no property advantages are seen in using the LLDPE.
Extrusion of these binary blends, even containing lower amounts of either polypropylene or polystyrene, results uniformly in products of unacceptable appearance. Particularly, these products consistently exhibit melt fracture and a surface of irregularities which are striated or wavy in configuration. Most attempts at remedies of melt fracture are physical or mechanical in nature. For example, it has been proposed to vary die geometry and increasing dies gap. These are solutions which invoke other problems. Modification of die geometry is costly and increasing dies gap or processing temperature create the problem of bubble instability for the blown film processing.